Police said a former senior member of Croatia’s ruling party was detained Friday as part of a war crimes investigation, about two decades after media implicated him in some of the most brutal atrocities against ethnic Serbs.

Police spokesman Krunoslav Borovec didn’t identify the suspect in line with privacy laws, but state-run Croatian television showed Tomislav Mercep being taken into a police station on Friday. Mercep also was briefly an interior ministry official during the 1991 Serbo-Croatian war.

For years, Croatia claimed that only Serbs, who took up arms in 1991 to rebel against Croatia’s independence from Yugoslavia, committed atrocities in the war. But the country has began prosecuting its own for war crimes in the past few years, though Amnesty International said in a report Thursday the prosecutions are still to too slow and biased.

In the report the rights group also criticized Croatia for not prosecuting Mercep, despite “publicly available evidence” against him.

Police said Mercep was suspected of alleged war crimes against civilians committed from October through December 1991 in the capital, Zagreb, and at a field near the central Croatian town of Pakrac.

In 1992, independent media in Croatia said that Mercep commanded a paramilitary unit that allegedly detained, tortured and slaughtered Serb civilians, but he was never prosecuted.

In 1997, a man claiming to be Mercep’s subordinate told Feral Tribune weekly that they were seizing Serb civilians from Zagreb before taking them to Pakrac, where they would be killed.

Four years ago, five men who allegedly belonged to Mercep’s unit were convicted and sentenced by a local court for killing four Serb civilians in Pakrac.

Mercep’s unit also reportedly killed a Serb family — a man, his wife and their 12-year-old daughter — in Zagreb in the early days of the war. The incident remains a symbol of Croat brutality during the war.

Vesna Terselic, a human rights activist, said the investigation against Mercep, “although coming with a delay, is certainly welcomed.”